1. Field of the Invention
The present invention relates to a direct reduction process, and more particularly to a direct reduction method wherein a gas for secondary combustion is fed so as to burn a flammable gas generated from objects to be heated (hereinafter referred to merely as objects). The present invention also relates to a rotary hearth furnace for carrying out the direct reduction method.
2. Description of the Related Art
Conventionally, there is known a process where in a metal oxide or a mixture of a metal oxide, coal, etc. in the form of pellets, briquettes, or the like (hereinafter may generically be referred to as objects) is charged into a rotary hearth furnace to undergo direct reduction.
In the aforementioned rotary hearth furnace, heat is supplied to objects during a direct reduction steel-making process by a plurality of burners, such that the inside temperature of the face is maintained at 1200-1500.degree. C.
In connection with direct reduction of this kind, there is known a method wherein a metal oxide which contains a reductant is supplied to a rotary-hearth type furnace and heated therein (e.g. U.S. Pat. No. 4,622,905). In the technique described in the publication, diversified burners are employed, and the (furnace is operated by mixed combustion with pulverized coal such that luminous flames are generated at or near an equivalent ratio.
According to U.S. Pat. No. 4,401,214, air is fed to burners only in an amount required to burn gas generated in association with reduction. The publication describes that the aforementioned atmosphere can be established by the use of gaseous or liquid fuel, and preheated combustion air.
According to the aforementioned conventional art, in manufacture of direct-reduced iron, air is fed in an equivalent amount into a furnace through the use of burners.
Also, two-stage combustion is conventionally performed in, for example, steel heating furnaces and boilers for the purpose of reducing the amount of generated NOx. There is also known a method wherein a flammable gas, which is generated within a furnace and contains CO.sub.2 and H.sub.2, is led into a separate secondary combustion furnace provided outside the furnace and is completely combusted therein.
In the case where a furnace is operated while the inside temperature of the furnace is maintained constant, as described above, a large amount of a flammable gas which contains CO, H.sub.2, CH.sub.4, etc. is generated when objects are charged into the furnace, and the amount of the flammable gas decreases with time. This phenomenon has been known in the past, and it has been a general practice to feed such a flammable gas into a separate furnace and burn the gas therein. Since no attention has been paid to such a flammable gas, such a flammable gas is not utilized for heating objects.
FIG. 12 illustrates two-stage combustion practiced in, for example, boilers. As shown in FIG. 12, a burner 30 is provided in each zone of a heating furnace, and fuel is fed through a nozzle 31. Air for combustion is divided into primary air 32 which is supplied through a clearance around the nozzle and secondary air which is fed from locations surrounding the clearance. Generally, the ratio of the amount of the primary air 32 to the amount of the secondary air is set at 0.6-0.8 to thereby establish reducing-gas atmosphere in a primary combustion zone through the generation of CO and H.sub.2 ; the secondary air is fed from the locations surrounding the stream of the primary air 32 to thereby completely combust CO and H.sub.2 generated in the primary combustion zone. Accordingly, combustion progresses slowly, so that the amount of generated fuel NOx and the amount of generated thermal NOx reduce significantly. However, in both excess-air-regulated combustion and two-stage combustion, no attempt is made to burn a flammable gas generated from objects in the vicinity of the objects.